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SubjectRe: [00/41] Large Blocksize Support V7 (adds memmap support)
On Tue, Sep 11, 2007 at 04:00:17PM +1000, Nick Piggin wrote:
> > > OTOH, I'm not sure how much buy-in there was from the filesystems guys.
> > > Particularly Christoph H and XFS (which is strange because they already
> > > do vmapping in places).
> >
> > I think they use vmapping because they have to, not because they want
> > to. They might be a lot happier with fsblock if it used contiguous pages
> > for large blocks whenever possible - I don't know for sure. The metadata
> > accessors they might be unhappy with because it's inconvenient but as
> > Christoph Hellwig pointed out at VM/FS, the filesystems who really care
> > will convert.
> Sure, they would rather not to. But there are also a lot of ways you can
> improve vmap more than what XFS does (or probably what darwin does)
> (more persistence for cached objects, and batched invalidates for example).

XFS already has persistence across the object life time (which can be many
tens of seconds for a frequently used buffer) and it also does batched
unmapping of objects as well.

> There are also a lot of trivial things you can do to make a lot of those
> accesses not require vmaps (and less trivial things, but even such things
> as binary searches over multiple pages should be quite possible with a bit
> of logic).

Yes, we already do the many of these things (via xfs_buf_offset()), but
that is not good enough for something like a memcpy that spans multiple
pages in a large block (think btree block compaction, splits and recombines).

IOWs, we already play these vmap harm-minimisation games in the places
where we can, but still the overhead is high and something we'd prefer
to be able to avoid.


Dave Chinner
Principal Engineer
SGI Australian Software Group
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